Mobile radio communications systems generally operate through a repeater wherein a first radio transmits a signal on a first frequency to a radio repeater that receives and retransmits the signal on a second frequency. The retransmitted signal can be received by a second radio, or a plurality of other radios. Repeater systems may comprise one or more radio repeaters each operating on a distinct pair or radio transmit and receive frequencies. A single pair of transmit and receive frequencies is called a channel. The radio coverage area of a repeater system is called a radio coverage cell, or simply a cell. Therefore, a repeater system, or system, defines a cell, and vice versa.
The repeater system architecture is used in many types of communications services, including land mobile radio, cellular telephone, radio paging, radio data systems, among others. It is common that a radio network is deployed which comprises a plurality of repeater systems so as to extend the geographic radio coverage area beyond that which a single repeater system or cell can cover.
In the land mobile radio environment, repeater systems having more than one channel trunk the repeaters together. Such systems are called trunked radio systems. A trunked radio system communicates with land mobile radios that have frequency agile transceivers that can dynamically access any one of several channels in any one of several repeater systems. In such a system, a communications protocol is employed to identify radios, identify repeater systems, control system access, identify radio call destinations, and other functions. Communications protocols have been developed which utilize in-band data, sub-audible data, and tone sequences to convey protocol information. Several protocols have become popular in the United States. Regardless of the protocol used, similar functions are provided for.
Another aspect of land mobile radio systems is that they are often interconnected with the public switched telephone network, PSTN, so that radio users can place and receive telephone calls. This is accomplished by interconnecting one or more repeaters with the PSTN. Telephone calls can originate in either the PSTN or from a mobile radio. Calls placed in this manner are called interconnect calls.
Another aspect of multi cell radio systems is that individual cells are joined to form a network by linking the repeater systems together and employing a network controller to manage call traffic between cells. In this environment, a radio located in a first cell can communicate with a radio located in a second cell by communicating from a first repeater system to a second repeater system over a network link. Such an architecture provides a great deal of convenience to end users in that the radio communications coverage area can be integrated over a wide service area.
Land mobile radios are programmed to operate in the particular communications environment for which they are intended. Each system will have one or more radio channels and each radio intended to operate in that system must be programmed with the particular frequency pairs associated with each channel in each system. Further, it is typical that each radio monitors a home channel in each system for the purpose or determining if the system is accessible and to determine if a call is being placed to the radio. If a radio is programmed to operate in more than one system, then the radio will have one or more channels for each system together with a designated home channel for each system programmed into a memory within the radio.
With regard to determining whether or not a system is accessible, several techniques have been employed. In the most basic approach, a radio merely attempts to establish a call by sending a signal to a system and waiting for a reply signal. If a signal is received, then the system is accessible. If no signal is received, then the system is not accessible. Another approach in testing for accessibility is to monitor signals and/or messages being transmitted by a system. If a radio is able to receive signals or messages, then the radio can assume that the system is accessible for the purpose of placing a call. There are certain drawbacks to the foregoing access testing techniques. In the case of merely attempting to place a call, there may be a substantial period of time required, depending on the particular protocol employed by a system, to determine that a system is not available. In the case of simply monitoring a system for signals or messages, there is a possibility that another system is operating co-channel with a desired system and the radio is falsely identifying the co-channel system. A refinement of this technique is to employ a protocol wherein each system transmits a unique system ID code that can be received by a radio to surely determine which system is being received. Even in this last case, there is a possibility that the receive range of the radio is greater than its transmit range. If this is the case, then the radio may attempt to place a call, only to have its transmissions ignored by the system due to the reduced coverage range on the mobile transmit frequency.
In a more advanced protocol design, radios check-in to each system when a system ID code is received. Because the check-in procedure involves the sending of signals and messages in both directions between the radio and the system, there is reliable confirmation that communications are possible.
Within each system are a pool of available ID codes that are transmitted on signals to determine which radio initiates and/or is intended to receive a particular call. Also, certain ID codes may indicate what type of call is being placed or received. For example, dispatch calls, interconnect calls, data calls, or network calls may have specific ID codes which identify the call type. Alternatively, data transmitted in the system, other than ID codes may be used to convey information about call types. However, generally, some form of identification, such as an ID code is necessary on a shared system in order to provide call privacy and to manage traffic flow.
Communications systems and networks provide a rather broad array of service options to end users. Upon placing a call, the user selects the repeater system or cell to be used for a call, and also the ID code and call type to be placed. This general format allows the user to specify the radio system used and the destination of each call placed. This offers a large degree of flexibility, however it can be problematic. If the user would select the wrong system, for example, the radio may be out of range of the selected system and be unable to initiate radio contact. If the wrong ID code is selected, the user may not initiate the desired type of call to be placed. In a similar fashion, if a particular user's radio is intended to receive a call, it is important that the radio be tuned to the correct channel and be monitoring a system which is in radio coverage range and to which a call will be addressed in order to contact the intended radio.
In the conventional form of trunked land mobile radio, the user selects a repeater system, SYSTEM, and a GROUP, to enable communications. The GROUP concept is grounded in the traditional dispatch call group being selected. In actuality, the SYSTEM specifies the repeater system and home channel which are monitored by the radio, and the GROUP specifies the ID code or codes used for communications. The SYSTEM/GROUP call selection scheme has certain draw backs in the modern multi-cell communications system. Firstly, it assumes the user is aware of which radio system or systems are within radio coverage range at any given time. Further, it assumes that the user knows to select an appropriate ID code, which selects the call type and call destination for each SYSTEM selected.
For example, if a user is a service vehicle which is currently located in the same cell as its dispatch office, it would be best to select the local SYSTEM and a GROUP which utilized a dispatch ID code in that system. This is because dispatch calls are generally fast, efficient and have a low cost as compared to other call types. The foregoing scenario is generally illustrated in FIG. 1. 0n the other hand, if the service vehicle is located in a cell remote from the dispatch office's cell, and the cell is not linked into a network with the dispatch office's cell, then a SYSTEM and GROUP should be selected which specify the local repeater system and an ID code which establishes a telephone call to the dispatch office. Note that in this example, the call is destined to a telephone at the dispatch office and not a radio. This scenario is generally illustrated in FIG. 3. Further, if the dispatch vehicle is located in a remote cell which is linked to the dispatch office's cell, then it may be appropriate for the user to select a SYSTEM and GROUP that specify an ID code which establishes a private radio to radio call to the dispatch office, over a network link connecting the two cells. This scenario is generally illustrated in FIG. 2.
As can be imagined, in a very large network with a complicated network link topology, the SYSTEM/GROUP selection combinations can become rather numerous and complicated for end users to understand. Clearly there is a need to simplify the selection of call destination for users roaming through a plurality of systems in a communications network.
As discussed earlier, certain trunking system protocols have become popular in the United States due to their vast deployment. One such protocol is the LTR.RTM. protocol developed by the E. F. Johnson Company of Waseca Minn. The LTR.RTM. protocol has become a defacto standard in a portion of the land mobile radio industry. Several manufacturers make radios and repeater equipment that are compatible with the protocol.
The LTR.RTM. protocol utilizes a sub-audible data stream for channel management and ID code communications. All channels in a given repeater system carry sub-audible data and may be specified as a home channel to mobile radios. The data stream on each channel comprises data frames, each having a data message. The mobile radio transmits a similar message to the repeater system and the combinations of transmission of the messages allow control of the trunked radio system.
Uniden America Corporation has developed an advanced trunking protocol and system which is backwardly compatible with the LTR.RTM. protocol. The advanced protocol is called the ESAS.RTM. protocol and is capable of emulating the functions of the LTR.RTM. protocol in addition to adding new functionality directed to network services, among others. New commands are used in the ESAS.RTM. protocol for various information including unique ID codes for radios (UID), telephone numbers, alphanumeric status messages, and other kinds of information.
The ESAS.RTM. system also comprises a network switch which is located at the repeater system site. The switch interprets ESAS.RTM. commands and data and routes call traffic both intra-site and intra-network. In addition, the protocol is capable of inter-network call management.